Add tests and longer error explanation

src/test/ui/E0660.rs

@@ -7,10 +7,11 @@
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
+
 #![feature(asm)]
 
 fn main() {
     let a;
-    asm!("nop" "nop"); //~ ERROR malformed inline assembly
-    asm!("nop" "nop" : "=r"(a)); //~ ERROR malformed inline assembly
+    asm!("nop" "nop");
+    asm!("nop" "nop" : "=r"(a));
 }

src/test/ui/E0660.stderr

@@ -0,0 +1,15 @@
+error[E0660]: malformed inline assembly
+  --> $DIR/E0660.rs:15:5
+   |
+LL |     asm!("nop" "nop");
+   |     ^^^^^^^^^^^^^^^^^^
+
+error[E0660]: malformed inline assembly
+  --> $DIR/E0660.rs:16:5
+   |
+LL |     asm!("nop" "nop" : "=r"(a));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0660`.

src/test/ui/E0661.rs

@@ -0,0 +1,16 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(asm)]
+
+fn main() {
+    let a;
+    asm!("nop" : "r"(a));
+}

src/test/ui/E0661.stderr

@@ -0,0 +1,9 @@
+error[E0661]: output operand constraint lacks '=' or '+'
+  --> $DIR/E0661.rs:15:18
+   |
+LL |     asm!("nop" : "r"(a));
+   |                  ^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0661`.